Millennial- to Centennial-Scale Anti-Phase Relationship between the Westerlies and the East Asian Summer Monsoon Over the Last 3500 Yr and its Cultural Response Along the Silk Roads

Abstract

It is hotly debated whether the Westerlies and the East Asian summer monsoon (EASM) are in-phase, anti-phase or out-of-phase. In this study, we established a 3500-yr-long paleoclimatic sequence based on ~ 8-yr resolution sedimentological and geochemical data of a precisely dated sediment core from Yileimu Lake in southern Altay. This new record indicated two wet intervals from 3500–2300 and 600–100 cal yr BP, interrupted by a severe and prolonged dry interval from 2300–1000 cal yr BP and a mild dry interval from 1000–600 cal yr BP. These results are broadly consistent with other paleoclimatic records in Westerlies-dominated Asia and are generally anti-phase with those in the EASM region. We suggest that a strengthening/weakening and southward/northward migration of the Westerlies during a negative/positive phase of North Atlantic Oscillation (NAO) transported more/less water vapor into arid Asia. Meanwhile, a decreasing/increasing El Niño-Southern Oscillation (ENSO) superimposed on a southward/northward shift of the Intertropical Convergence Zone (ITCZ) triggered a weakening/strengthening and southward/northward movement of the West-Pacific subtropical high (WPSH), resulting in the decreasing/increasing EASM intensity. In addition, the wet climate from 3500–2300 cal yr BP may have contributed to the development of nomadic herding in the eastern Eurasian Steppe and Altay region, and to the opening of the proto-Silk Roads. Potentially great seismic activities in the Altay Mountainous before 3500 cal yr BP may have also promoted the opening of the proto-Silk Roads by forcing the herdsmen to move to the inter-mountainous basins. The data in this study imply an increasing drought risk in arid Asia with current global warming, which may force local herdsmen to move to the monsoonal areas along the Silk Roads, and trigger cascading socio-economic responses.

Keywords: Westerlies, East Asian summer monsoon, North Atlantic Oscillation, El Niño-Southern Oscillation, Intertropical Convergence Zone, Silk Roads